1. Field of the Invention
This invention relates to a novel improvement in the process of preparing 2,6-di-tertiarybutyl-4-mercaptophenol by a zinc-acid reduction of bis(3,5-di-tertiarybutyl-4-hydroxyphenyl)polysulfide in the presence of a catalytic amount of lead. The addition of catalytic amounts of lead to the zinc reagent increases the reduction efficiency of the above reaction. The above process represents an intermediate step for making 4,4'-isopropylidenedithio-bis-(2,6-di-tertiarybutylphenol), which has been described in U.S. Pat. No. 3,576,883 as an effective pharmaceutical agent for the reduction of serum cholesterol and has been approved for marketing in the United States by the Food and Drug Administration (FDA).
2. Description of Prior Art
The reduction of bis(3,5-di-tertiarybutyl-4-hydroxyphenyl)polysulfide in the presence of zinc/HCl is disclosed in U.S. Pat. Nos. 3,952,064 and 3,479,407 and in Japanese patent application No. 73-28425. No mention, however, is made of the utility of a catalytic amount of lead in this reaction. U.S. Pat. No. 3,479,407 discloses the preparation of bis(3,5-di-tertiarybutyl-4-hydroxyphenyl)polysulfide by sulfurization of 2,6-di-tertiarybutylphenol with sulfur monochloride in the presence of an iodine catalyst. The Polysulfide can then be reduced to the corresponding mercaptophenol, i.e., 2,6-di-tertiarybutyl-4-mercaptophenol, which can be condensed using acetone under acidic conditions to form 4,4'-iso-propylidenedithio-bis-(2,6-di-tertiarybutylphenol), as described in U.S. Pat. No. 3,576,883. This reaction sequence is as follows: ##STR1##
The Zn/HCl reduction of the Polysulfide (II) to the corresponding Mercaptophenol (III) is inconsistent, resulting in variable, incomplete reductions which require multiple cycles through the reductive step to obtain satisfactory yields. The present invention which comprises an improvement in this process results in consistent high-yield reductions of the Polysulfide (II) to the corresponding Mercaptophenol (III) and substantially improves the process of making 4,4'-isopropylidenedithio-bis-(2,6-di-tertiarybutylphenol).